Extrusion dies are used to form a desired product out of heated, molten thermoplastic material. The extrusion die is fed with the material at an inlet of the die and produces the desired product at an outlet of the die. The outlet has a shape that is a cross sectional profile of the final product.
Flat sheet extrusion dies typically comprise two or more die bodies that are secured together, thereby defining an outlet between a lip of each die body. An example of this type of extrusion die is disclosed in commonly-assigned Wilson et al. U.S. Pat. No. 5,494,429, which describes a two-piece die body that forms a sheet of substantially uniform thickness.
During extrusion, the thermoplastic sheet exiting the outlet is still in a heated state, and as such, may emit small quantities of vapor or liquid that can cool, harden, and become affixed to either of the die lips in a manner somewhat analogous to the formation of an icicle. This hardened and affixed thermoplastic is sometimes referred to as bearding because the hardened thermoplastic may resemble whiskers extending from the lips of the die into the path of the extruded material exiting the die outlet. This bearding is undesirable because it tends to scrape against the extruded material, thereby creating die lines on the material. As a result, attempts have been made in the art to prevent accumulation of bearding on the die lips without having to manually scrape away such bearding. For example, Spencer U.S. Pat. No. 3,502,757 discloses an air sweep nozzle 22 that extends across roughly the entire width of the die. The nozzle 22 blows air toward one of the die lips to reduce or entirely eliminate build-up of material (i.e., bearding). Hazarie et al. U.S. Pat. No. 5,391,071 discloses an apparatus for cleaning first and second lips 22, 24 of a casting die that produces a thin polymeric film. Solvent nozzles 28 and 34 are disposed near the respective lips 22 and 24, and, upon detection of agglomerations of polymer on the lip, the nozzles 28 and 34 traverse the width of the die directing a stream of solvent at the lips 22, 24 to wash away any such agglomerations.
While numerous prior art dies incorporate cleaning apparatus that direct air or liquid at die lips to wash away debris, no known attempts have been made in the art to provide a lip cleaning apparatus having a scraping surface pressed against a lip of an extrusion die that scrapes debris off of the lip.